Heat exchanger and a charge air cooling method

ABSTRACT

The invention relates to a heat exchanger ( 1 ) for a charge air cooling, wherein water, in particular, condensation water in the form of droplets and/or fog is supplied and the heat exchanger ( 1 ) is provided with a hydrophobe surface in at least one partial area thereof.

The invention relates to a heat exchanger according to the preamble of claim 1 and to a method for cooling charge air according to the preamble of claim 11.

In order to increase the performance of engines, turbochargers are used to compress the air. Here, however, the air, referred to in the following as charge air, is heated to temperatures of over 150° C. as a result of the compression in the turbocharger. In order to reduce such heating of the air, air coolers are used which are arranged at the front of the cooling module and serve to cool the charge air. Here, the charge air flows through a heat exchanger which has ambient air flowing through it, and said charge air is thus cooled. This makes it possible to cool the charge air to a temperature which is approximately 20-90 K above the temperature of the ambient air. Cooling the charge air permits an increase in engine performance.

A two-stage device for cooling charge air and a method for operating such a device is known, for example, from DE 102 54 016 A1, said device permitting a further increase in performance as a result of improved charge air cooling.

A method and a device for operating a supercharged internal combustion engine are known from DE 28 14 593 C2, said method and device being used to accumulate the condensation water precipitated in the charge air cooler, discharge said condensation water out of the charge air cooler, accumulate said condensation water in an accumulation tank arranged separately from the charge air cooler, and supply the accumulated condensation water into the exhaust line of the internal combustion engine upstream of the exhaust gas turbine in the flow direction. For this purpose, a pump or at least a sufficient pressure drop is provided which conveys the condensation water from the accumulation tank into the exhaust line.

Heat exchangers which are used to cool air are provided with a hydrophilic coating in order to better discharge the condensation water which is accumulated from said air, since it is conventionally sought to avoid liquid water components in the cooled air.

Heat exchangers of said type, however, leave something to be desired.

It is an object of the invention to improve a heat exchanger of the type mentioned in the introduction.

Said object is achieved by means of a heat exchanger having the features of claim 1 and by a method having the features of claim 11. Advantageous embodiments are the subject matter of the subclaims.

According to the invention, a heat exchanger is provided for cooling charge air to which water, in particular condensation water, can be added in the form of droplets and/or mist, the heat exchanger having a hydrophobic surface at least in a partial region. By providing a hydrophobic surface in contrast to the known provision of a hydrophilic surface which is effective in conveying the condensation water downward under the force of gravity, the condensation water accumulates on the hydrophobic surface in the form of droplets, with the droplets projecting into the flow duct, and therefore being easily entrained and ultimately separated and being conveyed in the charge air flow in the form of droplets and/or mist. Here—in contrast to the prior art—no specific device is required to add condensation water to the charge air, in order to supply the condensation water in the form of droplets or mist to the charge air flow. The condensation water which is fed back to the charge air cools the charge air and therefore contributes to an increase in engine performance.

In contrast to conventional discharging of the water which is condensed out, targeted admixture of water takes place according to the invention, with the result that the cooling power can be increased, said conventional method being dispensed with. Conventional cooling of charge air by means of a second cooling stage, whose cooling power is, for example, generated by a refrigerant circuit, is therefore associated with considerable impairment of the efficiency of the cooling system. On the other hand, it is however undesirable to supply the water which is condensed out to the motor vehicle engine in an uncontrolled fashion, that is to say inhomogeneously in terms of space and time.

In the region of the hydrophobic surface, the contact angle of a droplet is preferably greater than 90°, preferably greater than 120° and particularly preferably greater than 150°, so that the condensation water accumulates on the surface in the manner of a pearl and can be easily entrained by the charge air flow. The hydrophobic surface permits the formation of approximately spherical droplets which are transported and entrained by the charge air flow when they are of a small size.

Separation edges are preferably provided on the heat exchanger, at which separation edges the droplets which have collected on the hydrophobic surface detach from the heat exchanger as a result of the prevailing charge air flow. Here, the separation edges are preferably also provided with a hydrophobic coating, so that the low adhesion forces allow the droplets to be easily detached from the surface. The separation edges are preferably formed by ends of web fins or gills of gill fins.

The detachment is supported by flow speeds of preferably over 3 m/s, particularly preferably over 6 m/s, for which the heat exchanger is correspondingly designed in terms of flow. High flow speeds additionally assist in the residence times on the hydrophobic surfaces being short, which can prevent a plurality of droplets coalescing, and makes the droplet size at the time of separation smaller.

To assist the formation of droplets, the heat exchanger can be electrostatically charged at least in a partial region of the surface, so that the droplets which are formed impact against one another as a result of the electrostatic charge and can, as a result, detach from the fin structure of the heat exchanger more easily. In addition, the tendency is reduced for the droplets to be trapped again by heat exchanger structures arranged in the flow direction. Electrostatic charging of the droplets also prevents said droplets joining together in the air flow, so that the droplets do not coalesce to form larger droplets. Here, the tendency for the droplets to be trapped again by subsequent fin structures is considerably greater for larger droplets as a result of the larger inertial forces, so that it is desirable for the droplets to be as small as possible.

The hydrophobic surface preferably has dispersing, electrically conductive constituents, for example in the form of nanoparticles which permit electrically conductive contact between the charged hydrophobic surfaces and the droplets rolling over the hydrophobic coating, so that the electrical charge can be better transmitted to said droplets.

As an alternative to the electrically conductive constituents, the heat exchanger can have at least one region with a neutral or hydrophilic, electrically conductive surface which permits electrostatic charging of the droplets. Here, the hydrophilic region is preferably considerably smaller, than the hydrophobic region.

The fins of the heat exchanger preferably have a spacing of a maximum of 2 mm, in particular a maximum of 1.5 mm, and can therefore be situated considerably closer together than the fins of conventional heat exchangers.

Good distribution of the droplets in the charge air is achieved by the fins having separation edges with a spacing of a maximum of 5 mm.

One preferred embodiment involves combining the features of a hydrophobic surface with mechanical vibration generation, preferably in the inaudible ultrasound range.

In a further embodiment, it is proposed to couple a vibration transducer to the evaporator, with the aim of detaching the condensate droplets which form primarily on the transmitting face of the heat exchanger from the surface by means of mechanical vibrations, and if appropriate, of separating said droplets into smaller droplets.

The vibration direction of the vibration transducer which couples vibrations in is preferably selected such that it is aligned perpendicular to the heat transmitting face. In a further embodiment of the concept, at least two vibration transducers are coupled to the evaporator, said vibration transducers being distributed locally such that the body-borne noise vibration field which permeates the evaporator is as homogeneous as possible and/or said vibration transducers complementing one another in terms of their vibration direction and phase position such that circular body-borne noise vibration is generated. This makes it possible for all the heat transmitting faces to vibrate with a vibration component perpendicular to the surface.

In a further embodiment, the vibration transducer can be adapted in terms of its frequency and amplitude such that resonant effects occur which preferably detach droplets of a particularly certain size from the surface. As a result, the power of the required ultrasound transducer can be limited to small values, and the detachment of small droplets can be assisted. In addition, the frequency and arrangement of the one or more vibration transducers in combination with the mounting of the heat exchanger and/or the connection of further noise conducting components can be adapted in terms of impedance in such a way that stationary waves with a particularly advantageous amplitude distribution are generated.

In a further embodiment of the concept, the vibration generation can also be utilized to increase the heat transfer coefficient, or to reduce the pressure loss, on the inside of the heat exchanger (that is to say the other fluid side). In evaporators in particular, the formation of bubbles can be assisted and/or laminar viscous underlayers can be broken up by means of cavitation effects. This could prove to be particularly useful in the evaporation of multi-component mixtures (for example refrigerant/cooling oil).

Coupling mechanical vibrations into the heat transmitting structure causes condensate droplets which form on the surface to be detached at least at times, and as a result permits the gas flow passing through the structure to be discharged out of the structure faster.

The invention is explained in detail in the following on the basis of two exemplary embodiments and with reference to the drawing, in which:

FIG. 1 is a greatly enlarged schematic illustration of a partial region (gill fins), which is provided with a coating according to the invention, of a heat exchanger according to the first exemplary embodiment,

FIG. 2 is a greatly enlarged schematic illustration of a partial region (web fins), which is provided with a coating according to the invention, of a heat exchanger according to the second exemplary embodiment, and

FIG. 3 is an enlarged schematic illustration of a heat exchanger according to the third exemplary embodiment.

According to the first exemplary embodiment, a heat exchanger 1 for cooling charge air which is supplied to a motor vehicle engine has a structure, which is known in principle, with gill fins 2 which are arranged obliquely and parallel to one another, with FIG. 1 illustrating only a greatly simplified and enlarged section through part of the gill fins 2. According to the invention, the gill fins 2 are provided with a hydrophobic surface coating which has the effect that the condensation water which accumulates on the gill fins 2, said condensation water accumulating out of the charge air on the cooler surface of the heat exchanger 1, accumulates in the form of droplets, as indicated by approximately circularly illustrated droplets 3 in FIG. 1. Here, the droplets 3, which have accumulated on the hydrophobic surface of the heat exchanger 1, have a contact angle of more than 90° relative to the surface of the heat exchanger 1, so that they roll off the surface of the heat exchanger 1, are entrained by the charge air flow, indicated by arrows, along the faces of the gill fins 2 and—after separation at a separation edge 4—are conveyed with the charge air flow as condensation water mist 5. Here, the charge air flows in the region of the separation edges 4 at a flow speed of over 6 m/s, so as to ensure that the droplets 3 are separated and entrained.

Since the entrained condensation water in the charge air is evaporated again during the suction and/or compression process, the charge air is cooled further, with the result that the engine performance can be further increased, for example by increasing the injection quantity and by means of its timing.

According to the second exemplary embodiment illustrated in FIG. 2, the heat exchanger 1 has a structure, which is known in principle, with web fins 12 which are arranged parallel and offset relative to one another. Corresponding to the gill fins 2 of the first exemplary embodiment, the web fins 12 of the second exemplary embodiment are provided with a hydrophobic coating which ensures that the condensation water which accumulates on the relatively cool web fins 12 rolls off.

The function of the hydrophobic coating is the same as in the previously described first exemplary embodiment, so this is not described in any more detail, but the flow profile of the charge air is more uniform as a result of the shape of the fins, and the charge air is not deflected significantly by the web fins 12 which run parallel to the flow profile.

FIG. 3 shows, in the form of a section, a heat exchanger 21 having flow ducts 22, embodied here as tubes, through which a fluid 1 flows, and having fins 23 which are embodied here as corrugated fins. A vibration 24 aligned perpendicular to the heat transmitting face is generated by means of two vibration transducers (not illustrated) with vibration directions (excitation 1 and excitation 2 respectively) which lie substantially perpendicular to one another. The body-borne noise vibration field which permeates the heat exchanger 21 is homogenized as a result. 

1. A heat exchanger for cooling charge air to which water, in particular condensation water, can be added in the form of droplets and/or mist, wherein the heat exchanger has a hydrophobic surface at least in a partial region.
 2. The heat exchanger as claimed in claim 1, wherein, in the region of the hydrophobic surface, the contact angle of a droplet is greater than 90°, preferably greater than 120° and in particular greater than 150°.
 3. The heat exchanger as claimed in claim 1, wherein separation edges are provided on the heat exchanger.
 4. The heat exchanger as claimed in claim 3, wherein the separation edges are formed by ends of web fins or gills of gill fins.
 5. The heat exchanger as claimed in claim 1, wherein the heat exchanger can be electrostatically charged at least in a partial region of the surface.
 6. The heat exchanger as claimed in claim 5, wherein the hydrophobic surface has dispersing, electrically conductive constituents.
 7. The heat exchanger as claimed in claim 1, wherein the heat exchanger has at least one region with a neutral or hydrophilic, electrically conductive surface.
 8. The heat exchanger as claimed in claim 1, wherein the fins of the heat exchanger have a spacing of a maximum of 2 mm, in particular a maximum of 1.5 mm.
 9. The heat exchanger as claimed in claim 1, wherein the fins have separation edges with a spacing of a maximum of 5 mm.
 10. The heat exchanger as claimed in claim 1, wherein the heat exchanger is designed in terms of flow such that, in the operating state which requires the admixture of condensation water, the flow speed in the region of the separation edges exceeds a value of 3 m/s, in particular 6 m/s.
 11. The heat exchanger as claimed in claim 1, wherein the heat exchanger comprises at least one or two mechanical vibration transducers.
 12. A method for cooling charge air, the charge air flowing through a heat exchanger and the charge air having water in the form of condensation water added to it, wherein the air flowing through the heat exchanger entrains condensation water which forms on the heat exchanger.
 13. The method as claimed in claim 12, wherein the condensation water which is entrained in the heat exchanger is conveyed onward in the charge air in the form of droplets or mist.
 14. The method as claimed in claim 1, wherein the condensation water accumulates in the heat exchanger in the form of droplets on hydrophobic faces, with the droplets which form having a contact angle of greater than 90°, preferably greater than 120° and in particular greater than 150°.
 15. The method as claimed in claim 1, wherein in at least one operating state in which condensation water is to be added to the charge air, the charge air in the heat exchanger flows, at least in the region of separation edges, at a flow speed of over 3 m/s, in particular over 6 m/s.
 16. The method as claimed in claim 1, wherein mechanical vibrations are generated in the heat exchanger, in particular by means of at least one vibration transducer which is coupled to the evaporator.
 17. The method as claimed in claim 16, wherein the mechanical vibrations are aligned substantially perpendicular to a heat transmitting face of the heat exchanger. 